Increased use of consumer electronics such as cellular telephones, laptop computers and other portable devices, and the development of new technologies like electric vehicles (EV) has increased the demand for compact, durable, high capacity batteries. This demand is currently being filled by a variety of battery technologies including traditional lithium-ion batteries. However, lithium-ion batteries with liquid electrolytes pose leakage and flammability hazards. In addition, the plastic separator used with liquid electrolytes can be punctured by lithium dendrites. Lithium-ion batteries that attempt to obviate these problems with semi-solid polymer electrolytes are susceptible to short circuits through internal contacts between protruding rough-surfaced electrodes. Thicker polymer electrolyte films are not a suitable solution because the inherent low ion conductivity of known polymer electrolytes is limiting. Semi-solid and solid-state electrolytes are desirable because they provide inherent advantages in the fabrication of consumer batteries in a wide variety of shapes and sizes that are thinner, safer and more environmentally friendly.
Lithium polymer electrolytes have received considerable interest for use in solid-state batteries. These electrolyte systems are complex materials composed of amorphous and crystalline phases. It has been known since 1983 that the ion motion in polymer electrolyte occurs predominantly in the amorphous phase. Accordingly, the conventional approach to improving ionic conductivity has been to investigate conditions that either decrease the degree of crystallinity or increase the segmental motion of the polymer matrix. However, despite significant improvement, the use of modern lithium-ion batteries employing polymer electrolytes is still limited; polymer electrolytes have inherently low ion conductivities and are too soft to prevent puncture and shorting by lithium dendrites.
Some of the first true solid-state batteries were developed by Duracell in the 1970s using aluminum oxide (Al2O3) powder and lithium iodide salt (LiI) as the electrolyte material. See, U.S. Pat. No. 4,397,924 issued to Rea on Aug. 9, 1983 (Rea '924). The solid alumina-based (Al2O3) electrolytes provided two orders of magnitude greater conductivity than polymer electrolytes and are hard materials that are not subject to puncture by lithium dendrites. Lithium cations traverse the alumina matrix by a hopping mechanism instead of the mechanism of segmental rearrangements effective in polymer electrolytes. In the alumina/LiI electrolyte, lithium ions travel across the surfaces of alumina particles by hoping from oxide oxygen to oxide oxygen on the amorphous alumina surface. However, Duracell and other manufactures have virtually abandoned this technology. (Kluger K, Lohrengel M, Berichte Der Bunsen-Gesellschaft-Physical Chemistry Chemical Physics, 95 (11): 1458-1461 NOVEMBER (1991)). Poor particle packing, mechanical vibrations and shocks can diminish effective interparticle contacts that can result in reduced numbers of pathways for ion conduction. Large metal oxide-type particles afford reduced capability of ion solvation and yield large interparticle insulating void spaces upon packing. Nano-size metal oxyhydroxide-type particles with inherently greater surface areas and capability for ion solvation are desirable to overcome the poor interparticle contacts associated with known materials.
There is a need in the art for new solid-state electrolyte/separator with high innate conductivity and less susceptibility to loss of long-range particle-to-particle ion conduction. Higher ion conductivities can be achieved in the solid-state by employing nano-size metal oxyhydroxide particles that dissolve alkali metal salts, and that can more tightly pack together to maintain contiguous particle-to-particle ion transport.